The present invention relates to a display device comprising a pair of plastic substrates which are bonded together by a peripheral seal, and comprises cell spacers to space the plastic substrates apart at an active area, at least one of the substrates being provided with a functional layer, while an electro-optical material is present between the substrates.
Display devices of this type, such as LEDs and liquid crystal display devices, have been known for some time. An electron-optical material, for instance, a liquid crystal material, is provided between a pair of substrates. At least one of the substrates is provided with a plurality of electrodes. Voltages applied to the electrodes govern the optical properties of the material. By correctly applying voltages to an array of elements and using illumination in the case of liquid crystal devices (either externally, including incident light or internally), an image may be produced. At least one of the substrates is provided with a functional layer such as conducting patterns, for instance ITO, electronic switching devices such as poly-silicon or amorphous silicon and/or permeation layers.
The image rendition, in particular the color rendition of the display device, is dependent, inter alia, on the distance between the substrates. To provide an accurate spacing between the substrates, spacers are used between the substrates.
Typically, the spacers are spherical balls mixed with the liquid crystal material. Up to now, glass substrates have been the most frequently used substrates for use in such devices, for instance, liquid crystal display devices.
When use is made of plastic substrates, the flexibility of the substrates allows application of the liquid crystal display device in situations where glass substrates are not or less suitable, for instance, when the display is to be applied to a curved surface or must be freely bendable.
However, the flexibility of the substrates themselves leads to a number of problems, for example, the image rendition may be poor and the functional layers are susceptible to failure due to bending or other causes. Such failure leads to malfunctioning or even destruction of the device.
It is an object of the invention to mitigate one or more of the above-mentioned problems.
To this end, the display device is characterized in that the spacers are in the form of ribs or columns, and the seal line has a height of less than xc2xc of the height of the spacers.
For a proper image rendition, height variations, i.e. variations in the distance between the substrates at the active area, are to be minimized. Height variations are dependent, among other things, on the spacer density, i.e. the number of spacers per unit area. For flexible substrates, a number of problems occur. On the one hand, a relatively large density of spacers is required for small height variations, on the other hand, when using spherical spacers, the inventor has found that the required densities are so high that there is a large risk of clustering of spacers, which in itself may be a cause of height variations and of image intensity variations. Calculations show that when using column or rib spacers, the required density is substantially less and clustering does not occur, which substantially reduces the height and image intensity variations and thus improves the image quality. When the device is put under mechanical load e.g. pressure (which may happen when the device is bent, touched or is subjected to temperature differences) the maximum stress exerted by spherical spacers on the substrates (and thereby on the functional layers) is much higher than for rib or column spacers. Such high stresses may lead to the failure of the functional layers. Application of rib or column spacers thus adds to the mitigation of these problems. Within the scope of the invention, rib or column spacers are spacers in the form of ribs or columns, substantially flat at the contact areas, i.e. having a large radius of curvature at the contact areas with the substrates in respect of the height, i.e. at least five times the height.
It is logical to use a seal of roughly the same height and to use the same or similar spacers in the seal as those used in the active area. However, the inventor has realized that reducing the height to substantially less than xc2xc of the height in the active area has a considerably advantage, for it reduces the permeation of gases or gas-like materials (such as water or oxygen) through the seal. These materials attack the functional layers and/or the LCD material itself, thereby substantially reducing the lifetime of the device. Reduction of permeation thus increases the lifetime of the device. In LEDs, the problem is so severe that usually getter material is provided to getter any permeating gases. In liquid crystal display devices, permeated gases are believed to be the cause of the creation of bubbles in the LCD material, which is especially a problem in bendable display devices, where these bubbles occur more often and in which there is a greater risk of the bubbles accumulating.
The inventor has realized that the use of spacers in the seal is less suitable at such seal thicknesses because the stresses exerted on the substrates, especially when bending the device, can run up to a very high level, which can produce failure of functional layers and/or the rheological properties of the seal material during manufacture of the seal. The presence of spacers has a negative effect, which may lead to holes and cracks in the seal. Preferably, the seal line therefore does not comprise spacers.
The seal line (also called xe2x80x98the sealxe2x80x99 in this application) and the active area are preferably separated by an inactive zone, which has a minimum width of less than 3 mm. The flexibility of the substrates allows a relatively small inactive zone of less than 3 mm. Much larger inactive zones are customary for glass substrates. The small inactive zone increases the percentage of active area in relation to the overall size of the device. The minimum width is preferably more than 0.5 mm. The inactive zone also functions as a buffer zone in which material can be buffered. Such a buffer zone provides the capacity of expansion of the LCD material so as to compensate for temperature changes.
The inactive zone preferably comprises spacers having a height which is smaller than the average height of the spacers at the active area and is larger than the seal height.
This reduces the risk of cracking of the functional layers.
In such embodiments, the spacer height in the inactive zone preferably diminishes from the active area to the seal.
The diminishing from the active area to the seal further limits the bending of the substrates and thus further reduces the risk of cracking of the functional layers.
The seal is preferably at least partly made of a metal. A metal has a very low permeability to water and oxygen.
Thin metal layers can be applied at the position of the seal on both substrates, outside the region of electrical contact. The layers can be made to form a (hermetic) seal by means of e.g. pressure bonding, soldering, laser welding or ultrasonic welding.